Ultra Clean Coal (UCC): Coal that is washed, ground into fine
particles, then chemically treated to remove sulfur, ash, silicone, and
other substances; usually briquetted and coated with a sealant made from
coal.

Ultraviolet Rays: Radiation from the sun that can be useful or
potentially harmful. UV rays from one part of the spectrum (UV-A) enhance
plant life. UV rays from other parts of the spectrum (UV-B) can cause
skin cancer or other tissue damage. The ozone layer in the atmosphere
partly shields us from ultraviolet rays reaching the earth's surface.

Uncertainty Factor: One of several factors used in calculating
the reference dose from experimental data. UFs are intended to account
for (1) the variation in sensitivity among humans; (2) the uncertainty
in extrapolating animal data to humans; (3) the uncertainty in extrapolating
data obtained in a study that covers less than the full life of the exposed
animal or human; and (4) the uncertainty in using LOAEL data rather than
NOAEL data.

Unconfined Aquifer: An aquifer containing water that is not under
pressure; the water level in a well is the same as the water table outside
the well.

Underground Injection Control (UIC): The program under the Safe
Drinking Water Act that regulates the use of wells to pump fluids into
the ground.

Underground Injection Wells: Steel- and concrete-encased shafts
into which hazardous waste is deposited by force and under pressure.

Underground Sources of Drinking Water: Aquifers currently being
used as a source of drinking water or those capable of supplying a public
water system. They have a total dissolved solids content of 10,000 milligrams
per liter or less, and are not "exempted aquifers." (See: exempted
aquifer.)

Underground Storage Tank (UST): A tank located at least partially
underground and designed to hold gasoline or other petroleum products
or chemicals.

Unreasonable Risk: Under the Federal Insecticide, Fungicide, and
Rodenticide Act (FIFRA), "unreasonable adverse effects" means
any unreasonable risk to man or the environment, taking into account the
medical, economic, social, and environmental costs and benefits of any
pesticide.

Unsaturated Zone: The area above the water table where soil pores
are not fully saturated, although some water may be present.

Upper Detection Limit: The largest concentration that an instrument
can reliably detect.